1. Technical Field
The present invention relates generally to an improved data processing system and in particular to an apparatus for transmitting data over fiber-optic fibers. Still more particularly, the present invention relates to an apparatus for equalizing signals transmitted over fiber-optic cables.
2. Description of Related Art
In fiber-optic data systems using multi-mode fibers (MMFs), inter-symbol interference (ISI) due to modal and chromatic dispersion significantly limits the achievable transmission distance. At a given data rate, ISI must be reduced if transmission distance is to be increased. For example, in 10 Gb/s Ethernet links, it is desired to achieve the same 300 meter distance as in previous generation networks, but the increased data rate makes it difficult to do so unless the optical fiber is upgraded. An economic incentive is present to use lower-cost MMFs and especially to utilize the large installed base of fibers in existing local area networks (LANs), so it is desirable to find a cost-effective way to reduce the ISI.
While the fiber dispersion can be partially compensated by pre-emphasis or coding at the transmitter side of the link, receiver equalization is the most convenient method for dispersion compensation, since any technique used needs to be adaptive due to the temporal variation of the dispersion with laser launch conditions and temperature, as well as variation with fiber length.
The use of continuous-time, transverse-form analog finite-impulse-response filters (often referred to in the literature as “transversal” filters) for optical receiver equalization is well known in the literature. Less complex filters of similar architecture are also used for cable equalization. Due to the high data rates involved, most of these filters are implemented using SiGe BiCMOS or GaAs MESFET technologies.